The so-called "IC Card Electronic Audio Book" is an electronic device that reads information stored in an IC card and converts it into audio information to replace traditional books. It is designed for people who have difficulty with visual reading, such as the elderly, the blind, or patients.

At present, the capacity of IC cards sold in the domestic market can reach 500K bytes. If storing voice signals directly (at a sampling rate of 8K/s), it can hold about 62.5 seconds of direct audio information. Although the audio quality of direct storage is good, it is obviously insufficient to meet the large information storage requirements of ordinary books. The author uses a self-defined Chinese character phonetic coding format for storage, where each Chinese character only occupies 1.5 bytes (12 bits). In this case, a 500K-byte IC card can store no less than 333,000 Chinese characters, supporting continuous playback for nearly 30 hours, which fully meets practical needs.

The design scheme presented in this paper uses the AT89LV51 single-chip microcomputer as the microprocessor, the ISD4004 for audio support, and the AT45D041 IC card as the information storage carrier for the electronic book. The AT45D041 IC card is a high-capacity memory card (4M bits) launched in recent years by Atmel Corporation of the United States, and it is currently the largest-capacity IC card easily available on the market. With technological progress, when replacing it with other types of IC cards with larger capacities, this design scheme remains applicable as long as the communication method remains unchanged. Even if the communication method is altered, only minor modifications to the scheme are required.

The basic working process of the device is as follows: First, a Chinese character text file (such as *Romance of the Three Kingdoms*) is converted into a phonetic coding file and stored in the IC card (this work is completed by a PC); then, when the IC Card Electronic Audio Book is in operation, the CPU in the playback device reads the phonetic codes from the IC card; finally, the CPU controls the ISD single-chip audio recording and playback circuit according to these codes to play the corresponding audio.

This design scheme is a basic, economical and practical design for an electronic audio book. Although its audio quality is not as good as the scheme of direct voice storage, it has the advantages of low device cost, small size, low power consumption and long continuous playback time.

1 Hardware Design

The electrical schematic diagram of the playback device is shown in Figure 1.

Figure 1 Electrical Schematic Diagram of IC Card Electronic Audio Book

Six keys are set in this design, namely "Play" (BUTTON0), "Fast Forward" (BUTTON1), "Slow Forward" (BUTTON2), "Fast Rewind" (BUTTON3), "Slow Rewind" (BUTTON4) and "Stop" (BUTTON5).

To resume playback from the last pause position at the next startup, a power-down data memory is required, and the AT24C01A is used in this paper.

To minimize the volume and weight of the playback device, the device uses only one AA battery. Therefore, a step-up switching regulator is adopted, and the MAX856 is used in this paper. To save battery power, the device should have an automatic power-off function. When the "Play" key is pressed, pin 1 of the MAX856 chip is pulled high, and the device is powered on. Thereafter, the single-chip microcomputer sends a high-level pulse to pin 1 of the MAX856 chip every 16 seconds to keep the device powered. When the timing time (60 minutes) is up, the device stops sending high-level pulses to the MAX856 chip, and the device automatically powers off.

The main functions and technical parameters of the AT45D041 high-capacity IC memory card adopted in this design are as follows:

(1) SPI-compatible serial interface;

(2) 4,325,376 bits of storage capacity;

(3) Low power consumption, with a dynamic current of 15mA and a static standby current of 20μA;

(4) Maximum clock frequency of 10MHz.

For the pins and commands of the AT45D041 high-capacity IC memory card, please refer to Reference [1]. Pins 1 to 8 of IC_CARD in Figure 1 correspond to pins 1 to 8 of AT45D041; pins 9 and 10 are auxiliary contacts of the IC card holder, used to detect whether the IC card is inserted.

The main functions and technical parameters of the ISD4004 audio chip adopted in this design are as follows:

(1) 16 minutes of audio recording and playback, with a minimum segment length of 200～400ms (300ms is adopted in this design);

(2) Maximum 2400 segments;

(3) SPI interface mode.

The main pins of the ISD4004 audio chip and their connection methods with the single-chip microcomputer are described as follows: AUD OUT — Audio signal output. To obtain a larger output, a power amplifier LM386 is used for amplification before supplying to earphones or speakers. SS — Chip select, active low, high level between two instructions. MOSI — Serial input end, the main controller should place data on this end half a cycle before the rising edge of the serial clock. MISO — Serial output end. SCLK — Serial clock input end, generated by the main controller for synchronizing data transmission of MOSI and MISO. The above four terminals are connected to P1.7～P1.4 of the single-chip microcomputer respectively. /INT — Interrupt, this terminal is an open-drain output, which goes low and remains low when EOM or OVF is detected during any operation; connected to INT0 of the single-chip microcomputer. RAC — Row address clock, open-drain output for memory management, connected to P1.3 of the single-chip microcomputer.

2 Formation of Phonetic Code Files

The ISD4004 audio chip stores sound by segments, with one sound per segment and each sound lasting 300ms. However, it should be noted that many characters have the same pronunciation (e.g., ye3: 也, 冶, 野); while some pronunciations have no corresponding Chinese characters (e.g., wai2, sai3, ka2, etc.). For each different pronunciation, there are 5 basic tone variations (namely Yinping 1, Yangping 2, Shang sheng 3, Qu sheng 4 and Light tone 5) in sequence. As long as there is a Chinese character corresponding to it (only one is taken for the same pronunciation), a code is assigned to it, and thus all phonetic codes can be obtained. There are a total of 1311 existing pronunciations for Chinese characters. In the design, all different pronunciations are arranged by English bytes and tone order, resulting in a total of 1337 phonetic codes (1311 Chinese characters + 26 English letters), that is, 1337 sounds. One phonetic code (i.e., the pronunciation of one Chinese character) can be stored in 1.5 bytes, so a 4M-bit IC card can store nearly 349,500 Chinese character pronunciations. After the phonetic codes are determined, the 1337 sounds can be written into the ISD4004 audio chip in the order of the phonetic codes.

First, a conversion table between the internal codes and phonetic codes of the two-level Chinese characters specified in GB2312-80 is established, see Table 1. For each character in a given text file, it is only necessary to find out its position code, then obtain its Pinyin code through table lookup, and finally write the Pinyin code into the high-capacity IC card. Punctuation marks are processed as pauses (no sound). It is worth noting that some Chinese characters (e.g., 和, pronounced as he2, he4, huo2, huo4, hu2) have multiple pronunciations, and thus multiple phonetic codes correspond to these characters. Which phonetic code to convert to needs to refer to the preceding or following character of the Chinese character. If it is still uncertain, the pronunciation with the highest occurrence probability should be taken (e.g., for 和, he2 is adopted).

Table 1 Conversion Table of Position Code and Pinyin Code (Fragment)